Apparatus for generating electrical pulses



March 13, 1951 A. D. BLUMLEIN 2,545,018

APPARATUS FOR GENERATING ELECTRICAL PULSES Filed D90. 8, 1945 2Sheets-Sheet l m, 1 T T T Z March 13, 1951 2,545,018

A. D. BLUMLEIN APPARATUS FOR GENERATING ELECTRICAL PULSES Filed Dec. 8,1945 2 Sheets-Sheet 2 m agnetron; Y

Patented Mar. 13, 1951 APPA A'r sron GENERATING ELECTRICAL PULSES AlanDower Blumlein, deceased, late of Eali'ng, London, England, by DoreenWalkei', exeouti-ix, Lanherne, Lescudjack, Pen'zance, Cornwall, England,assignor to Electric; 85 Musical 'Indus,-' tries Limited; Hayes,England, a company of Great Britain Ap iicationneceinber s, 1945, SerialNo. 633,832

In Great Britain October 28, 1941 section l, Public Law 690. August 8,1946 Patent expires October 28, 1961 9 Claims; 1

This invention relates to apparatus mainly for the generation ofimpulsive waveforms of current or potential of a desired shape usingdelay networks. V 4

For enerating snort im u ses or electrical energy it has previousiv beenproposedtd use a delay "network fine connected at one end across theanode load of a valve and short-circuited at the other so as to producepulses of short durationfrom pulses of anode current of longer duration.purposes or purse modulati n it has been proposed to use an opehcircuited dc"- iietvvork, the iatter being charged to a form potentialalong its length and then being discharged through the road. in general,delay networks if composed ofnianv identical sections, produce impuls shaving a fiat-topped waveform. impulses of this part'icciar waveform areorten very desirabie, esneciaiiy when the pulses are r quired tomodulate a; device such as a magne tron oso'inator whose frequency isdependent upon the input current. ff, however, a magnetron is fed withpulses or fiat-topped waveform, such as 'afe'genrated by the fines abovereferred to,

has passed thi oughthe transformer a flattopped waveform results forapplication to the The object of the present invention is to provide animproved apparatus whereby waveforms of a desired shape can begenerated, the invention being suitable for use for overcoming orreducing the above-mentioned disadvantage and other purposes.

2 length and one end of Said network being misterminatd so as to providesubstantial renections from such end during operation whereby derivedvariations or impulses are generated or V a desired form.

Where it desired to apply to a load, such as a magnetron, impulsivewaveforms of a predetermined shape and said derived waveforms are fedfrom said network to said load through means which distorts the shape ofsaid derived waveforms, it is arranged that the derived waveiorms are ofsuch a shape that, after distortion by'said means, the waveforms are ofsaid prodetermined shape.

The invention mat also be employed where it is desired to control therate of discharge of a network at the beginning of a pulse so as tolimit the rate of rise of current in a discharge device, such as isknown by the registered trade- 'Ifhyra-tron.

According to another feature of the invention thef' 'fs providedapparatus for transmitting electrical signals, sai d apparatus cmprising means which dis-torts said signals and a delay network thecharacteristic impedance of which varies along its lengthand one end ofsaid network bein sotermina-ted as to provide substantial reflectionsfrom such end during operation, the arrangement serving to compensatewholly or in part said distortion.

In order that the said invention may be clearly understood and feadilycarried into efiec't, it will now be more fully described with referenceto the dcditiiiafi-yilig drawings, in which:

Figure 1 illustrates diagrammatically apparel tus in accordance with theinvention, i

Figures 2 and 3 are illustrative of the mode of operation of saidapparatus,

Figure 4 shows a typical impulsive waveform such may be generated bysaid apparatus;

a Referring toFigur'e- 1, there is showdforg'enerating impulses of adesired shape in a load, a delay network l0 wl'i'ose impedance Z variesalong its delay length in accordance with the invention. The network itis terminated at one end through an impedance M and a source or"potential G2 which is arranged to generate intermittently a waveform ofpotential or current and which feeds to in the electrical energy sogenerated. The impedance H is the sum of the load and the sourceimpedances and is arranged to be equal to the impedance presented to itby the network It and will be supposed to constitute the Whole of theimpedance terminating the network H]. The end of the network remote fromthe source I2 is as shown misterminated by an open circuit or a shortcircuit so as to provide substantial reflections during operation. Theimpedance of the network H! at anypoint may be represented by This mustbe the magnitude of the terminating impedance H. Now the characteristic.impedance of the network at any point is given by:

Suppose that the source of potential 12 genwhere erates across theterminated end of the network a rectangular impulse of, for convenience,unit magnitude indicated by 0, Figure 2, lasting for a very short periodAt, and suppose that when this impulse has reached the point on thenetwork II] where the impedance is to its position as shown where itsamplitude by a. process of continual transformation has become V. At theposition shown the impedance of the network is I In further time At theimpulse will have passed to the point where the impedance isCorresponding to this change of impedance a reflected wave will be setup of amplitude:

small a wave will be returned or scattered back to the terminated end ofamplitude:

there appears at the end S a continuous succession of impulses ofmagnitude 2( t ertrepresenting the time at -which each impulse arrivesat S. The distribution of potential between the position of the impulseat time and the source due to the'presence of the succession ofscattered impulses flowing back to the source along the network isindicated in Figure ,3 by the envelope r.

If instead of generating a very .short duration rectangular impulse asjust supposed, the source !2 generates a step potential of unitmagnitude, the step commencing at time i=0, then the source may beregarded. as emitting a continuous succession of short-durationrectangular pulses of unitramplitude. Each of these will give rise to acontinuous stream of reflected impulses back upon the source. At time tthe sum total of these scattered impulses present at the terminated endwill be that is to say, in the limit when At is made infinitesimallysmall, the integral we) L e),

The resultant potential difference developed across the impedance 1 Iwill thus be work if the impedance ll-represents the load, and thesource or effectivesourcell is of zero impedanceg v If the total delayof the network is "-theii'the' above formula 'for the reflected waveacross the terminated endof the network super} posed upon the incidentstep wave will hold generally for all values of t up to 'I. Since thenetwork is misterminated at the end remote from short-duration impulsesreflected back from the remote end will give rise on its way back to thesource to further small scattered impulses at each point along thelength of the network which will travel up to the misterminated end andthen reflect back to the source. since on the way back to the source thegradient of impedance along the network will be of reverse sign to thatfor propagation up the network, the reflected impulses travelling to themisterminated end will be'of opposite sign to those which are reflectedback to the source in the way first considered. Thus, at time t,subsequent to time T, if the mistermination gives rise to iii-phasecomplete renectlon, the total reflected Waveform b scattering, omittingthe end reflection of the main step, is the sum z o)? ear 'As Atapproaches 0, this becomes I l 42 "Li i i T $4 2 y, K2) 2).

After time 215 the sum of all the small contributions reflected fromalong the length of the net- I I work becomes zero, and a steady stateis set up.

This statement is true if second'o-rder reflection set up by scatteringfrom each of the small first order reflections alonetaken into accountin the analysis are so small as to be negligible.

, Subject to these conditions holding the resultantwave-form ofpotential developed across the impedance 11 will be given by Wit)" andas before the impedance H may be a load impedance fed from said network.

- The above analysis has been developed for potential waves, but theformulae obtained hold equally well for current waves if the sign of thescattered waves is changed; Thus-for current waves in the above formulaewhere the terms r oal occur, the signs of these terms will be reversed.Figure 4 shows a Waveform conforming to the 1 log log analysisgivenabove, the waveform being that of 6 of impedance along the network isone that-is comparatively flat, that is to say, slowly varying, If thedistribution of impedance departs largely irom'this form, it will benecessary to take into account reflections of higher order than thefirst. It is believed, however, that the form of ulse obtained byignoring second and higher order refiections is accurate for timesbetween 0 and T for comparatively large deviations of impedancedistribution from the slowly varying form. By examining the discrepancybetween the calculated ower dissipated in the load and the charge storedin the condensers it would appear that second and'higher orderreflections will add terms where n is the order of the reflection, theseterms being necessary to give correctpower conditions. It would,-further, appear that the even-order re= flections cannot alter the waveshape during the period '0 to '1" so that for this eriod the error inthe analysis given above begins for third order reflections,

Figure 5 shows a waveform generated as described above: wherein theotherwise matched loadis shunted by the inductance of a transformer andit is required that the current to the load,assumed to be resistive,should be of fiattopped waveform, despite the magnetis'ing current takenby the transformer. Curve i'r'epr'esents the current pulse shaperequired through the load; curve 2 re resents the magnetisihg current ofthe transformer; and curve 3 represents the required current output ofthe apparatus according to the invention, Since the voltage develo edacross the load is to be constant, it follows that the network should beso designed as to give an output into a matched load such as thatrepresented by curve- 4, namely, a mean between curve I and curve 3. Forsmall departures from the" distribution of impedance Where the impedanceis constant along its length a network designed to generate a pulserepresented by the mean curve 4 should give the required currentwaveform If the curve 4 is represented by oft) where t is the t me fromthe start of the pulse, then equating oft) to the output wave calculatedaccording to the analysis given above, an equation for the impedancedistribution of the network is given as and thus This defines the ratioor characteristic imped anceat the point where the delay is ;quired'load voltage to the mean total required current, thatis' to say,thecurrent re resented by-the mean ordinate of curve 3'. If the requiredwa;veform isone of potential the above formula maybe used with the signof the-index of the exponential changed to be positive/ N Y j Figures 6and '7 illustrate two embodiments of theinvention where amodulatingpulse is applied through a transformer IE to a high frequency oscillatorsuch as a magnetron 11.:

In Figure 6 a stepped wave is applied periodically to the network ill bymeans of a gas or --vapor tube-E 2, such as a thyratron, and a voltagesource 12a. A trigger pulse applied to the grid of tubei2 causes it tobreak down, thus apply- ;ing the stepped wave. The reflected voltage ofreversed polarity extinguishes the tube I2.

In Figure 7 a direct-current voltage is applied by way of a choke coilIt to the network ill to charge it. The network I ii is dischargedperiodically through a thyratron or'the like indicated at I 9.

The invention is also of application where a thyratron or other deviceliable to damage by excessive initial rates of rise of current there--through is used to discharge a delay network which aifords a source ofenergy for the thyratron. If a flat-topped current pulse output isrequired from the network it is advantageous to subdivide the networkinto very many sections. 'In, these circumstances, when the thyratronfires, the rate of rise of current in the thyratron 'is liable to bevery rapid and thus likely to damage the thyratron. This rapid rise canbe pre- .vented by inserting an inductance between the network and thethyratron so as to limit the rateof rise of current to the prescribedsafe value .for the'thyratron. However, such an inductance causes thewave front of the generated impulse .to approach-the flat-toppedcondition exponentially so'that a long period must elapse before asubstantially flat-topped wave is obtained. This difliculty can beavoided according to the invention by inserting a delay network composedof a number of graded sections between the said inductance and thenetwork which provides the energy source. These sections will compriseinductances that diminish exponentially from the value of the firstinductance to preferably the value of inductance employed in the networkwhich provides the source, the time delay of all sections both of thelatter network and graded network being chosen to be the same. A currentwave will then be obtained which rises more or less uniformlytowards the maximum current through the thyratron. In practice a trueexponential distribution is not strictly correct, but serves as a guide,the values being adjusted if desired by trial and error to give therequired waveform through the thyratron. In this example of theinvention it will be appreciated that the load and source are disposedat opposite ends of the line with'the load at the misterminated end.

The manner in which the invention functions in the example justdescribed may be understood as follows: when the thyratron fires acurrent impulse of step form commences to propagate along the insertednetwork from the end at which the thyratron is connected towards that atwhich the network which provides the source is connected, As the impulseproceeds up the tapered line it encounters. a continually decreasingimpedance. Fromevery point along the network which the impulse haspassed there flows back therefore to the thyratron a reflected currentof like'sign to the propagating impulse. The current in the "thyratronwhich initially is small on account of the large mismatch between itsown low impedance andthe impedance of the tapered network presented toitis thus built up by an increasing number of increments byre flectionalong the tapered network. These increments are not equal to thereflected currents reflected back to the thyratron but are thetransmitted fraction thereof permitted by the substantial reflectionthat occurs at the thyratron termination by reason of said largemismatch. The refiected currents at the thyratron termination arehowever returned repeatedly by reflection along the length of thetapered network and each time in such sense as to augment the thyratron?current, and this continues until all reflections have died out.-- Bysuitably choosing the distribution of impedance along the taperednetwork the increments fed to the thyratron due to the resulting systemof reflections may be made to build up the thyra tron current in a safeand desired manner, it being appreciated that owing to the mismatch ofimpedance between the thyratron and the tapered network and theconsequent substantial reflections which thereby result the currentwhich is initially applied to the "thyratr-on when the latter fires andbefore the steady state is reached is reduced compared with what thecurrent would have been if there were no tapered network and if atapered network were used which was matched to the thyratron, thisreduction in current being desired for the safe operation-of thethyratron.

It will be understood that although the invention has been describedespecially with regard to generating fiat-topped or sloping-toppedpulses, curve-topped pulses may be developed from the network havingvaried constants along its length. The invention is also not limited tothe application to the network of step -type pulses, since other shapesof pulses may be applied to the network.

The term delay network used herein is intended to include a cable usedas a delay nets work.

The term vapor tube used in the claims is intended to include tubesfilled with either gas or vapor.

What is claimed is:

1. A pulse producing circuit comprising a waveforming delay networkhaving a plurality of sections, each section having series inductanceand shunt capacitance, said network having a reflecting termination atone end and a non-refleeting termination at the other end, a loadcircuit connected to said non-reflecting end, means for changing anelectricalcharge on said network periodically whereby periodic pulsesare supplied to said load circuit aplurality of said sections havingdifferent surge impedances than the remaining sections whereby a pulseis obtained that departs from a flat-topped wave form in-accordance withthe adjustment of said plurality of sections.

2. The invention according to claim 1 wherein the surge impedance ofeach section increases progressively from one end of the network to theother end of the network whereby a-pulseis obtained that has a slopingtoo.

3. Apparatus according to claim 1 wherein said means for changing anelectrical charge and said load circuit are connected to one end of.said network and wherein they have impedance values such as to terminatesaid network in its own impedance at said one end.

4. Apparatus according to claim 3 wherein said means for changing theelectrical charge comprises means for applying energy of stepped waveform to said network and wherein the characteristic impedance of saidnetwork slowly varies along its length and wherein for the purpose ofdeveloping across said impedance a waveform represented by (t) at time tafter the application of a variation or impulse, the ratio of thecharacteristic impedance of said network at any point distant from theterminated end of the network by a delay time to the surge impedance atsaid terminated end is:

where the positive or negative sign is taken according as said derivedwaveform is of current or of potential respectively, where (0)represents the wave form at the terminated end of the network, and wheree is the base of natural logarithms.

5. Apparatus according to claim 4 wherein the characteristic impedanceof said network increases substantially exponentially from said one endto said other end.

6. In combination, a wave-forming delay network comprising a pluralityof sections each having series inductance and shunt capacitance, adirect-current source for charging said network, a load circuit, meansincluding a vapor tube for discharging said network periodically throughsaid load circuit, a plurality of said sections hav-' ing differentsurge impedances than the remaining sections whereby a pulse is obtainedthat departs from a fiat-topped waveform in accordance with theadjustment of said plurality of sections.

7. In combination, a wave-translating device having the characteristicthat a flat-topped voltage pulse applied to its input circuit appearsacross its output circuit with the top of the pulse distorted in shape,and means for applying to said input circuit a pulse having its topdistorted in a shape substantially complementary to said first-mentioneddistortion whereby a flat-topped pulse appears across said outputcircuit, said last means comprising a wave-forming delay network havinga reflecting termination at one end and comprising a plurality ofsections each having series inductance and shunt capacitance, and a loadcircuit including said wave-translating device connected across theother end of said network and terminating said network substantially inits surge impedance, a plurality of said sections having different surgeimpedances than the remaining sections whereby a pulse is obtained thatdeparts from a fiat-topped waveform in accordance with the adjustment ofsaid plurality of sections.

8. In combination, a transformer having the characteristic that aflat-topped voltage pulse applied to its primary appears across itssecondairy with the top of the pulse sloping in one direction, and meansfor applying to said primary a pulse having its top sloping in theopposite direction whereby a flat-topped pulse appears across saidsecondary, said last means comprising a waveforming delay network havinga refleeting termination at one end and comprising a plurality ofsections each having series inductance and shunt capacitance, adirect-current source for charging said network, a load circuitincluding said transformer connected across the other end of saidnetwork and terminating said network substantially in its surgeimpedance, means including a vapor tube for discharging said networkperiodically through said load circuit, a plurality of said sectionshaving difierent surge impedances than the remaining sections whereby asloping-topped pulse is applied to said primary.

9. A pulse producing circuit comprising a waveforming delay networkhaving a plurality of sections, each section having series inductanceand shunt capacitance, said network having a reflecting termination atone end and a non-reflecting termination at the other end, a loadcircuit connected to said non-reflecting end, means for changing anelectrical charge on said network periodically whereby periodic pulsesare supplied to said load circuit, one of said sections having adifierent surge impedance than the remaining sections whereby a pulse isobtained that departs from a flat-topped waveform in accordance with theadjustment of said one section.

DOREEN WALKER, Erecutrix of Alan Dower Blumlein, Deceased.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,018,320 Roberts Oct. 22, 19352,179,607 Bedford Nov. 14, 1939 2,188,970 Wilson Feb. 6, 1940 2,227,021Schlesinger Dec. 31, 1940 2,420,302 Darlington May 13, 1947 2,420,309Goodall May 13, 1947

